This invention relates to aqueous cleaning compositions and methods of using same to remove rosin flux residues from substrates. More particularly, this invention relates to aqueous cleaning compositions containing alkaline salts and a particular surfactant formulation which includes fluorinated surfactants, and methods of using such compositions to remove rosin flux residues from electronic circuit assemblies.
In the electronics industry, electrical components such as resistors, capacitors, inductors, transistors, integrated circuits, chip carriers and the like, are typically mounted on circuit boards in one of two ways. In one way, the electronic components or modules are designed to mount to the printed circuit boards (PCBs) by means of plated through-holes in which the metal leads of the modules are spaced apart and sized to fit into corresponding plated through-holes and extend a small distance beyond the undersurface of the PCB. An alternative to the through-hole technique for mounting electronic modules on PCBs is surface mount technology (SMT) wherein the leads of electronic modules are soldered to metal pads plated on the surface of a printed wiring board. In this technique, a solder paste is applied to the metal pads and subsequently the electronic components are precisely placed on the PCB such that the coplanar leads of the module contact corresponding pads on the circuit board which are coated with a layer of solder paste. The solder paste comprises a soft solder alloy typically in a powder form and dispersed in a liquid medium conventionally containing a fluxing composition, an organic solvent and a thickening agent which provides the desired viscous or paste-like consistency to the solder formulation. The solder paste typically has sufficient adhesive strength to hold the components in position until the solder is melted. After application of the solder paste and placement of the electronic components, the entire PCB assembly is heated in a reflow oven to melt the solder in the solder paste thereby forming solder joints which permanently affix and electrically connect the electronic modules to the PCB. The assembly is then washed to remove the flux residue and tested.
If not carefully removed after the soldering process, traces of the soldering flux residues which remain on the boards can lead to circuit failure. Soldering fluxes fall into three broad categories: rosin fluxes, water-soluble fluxes, and no-clean fluxes. Rosin fluxes, which have a relatively long history of use and are still widely used in the electronics industry, are generally only moderately corrosive. Water-soluble fluxes, which are a more recent development and which are increasingly used in consumer electronics, are highly corrosive materials. No-clean fluxes, a very recent development, reportedly do not require removal from the circuit assemblies. However, residues of any flux are believed to cause circuit failure if residual traces of the material are not carefully removed following soldering and, thus, remain on the electronic circuit assembly. Certain circuit board assemblers even require the removal of no-clean flux residues from circuit boards.
Water-soluble fluxes are relatively easy to remove from electronic circuit assembly substrates by means of water or warm detergent solutions. However, recent SMT has yielded boards having a very high density of components thereon, rendering the cleaning of even water soluble fluxes problematic. A particularly useful cleaner for removing water-soluble flux residues is described in copending, commonly assigned U.S. patent application Ser. No. 08/391,015, filed Feb. 21, 1995. The cleaner disclosed in such application is capable of removing water-soluble fluxes from very densely configured circuit boards and can remove white residues which have also been problematic when using water-soluble fluxes.
Rosin fluxes are difficult to remove. Traditionally, rosin fluxes have been removed from printed circuit boards by means of chlorinated hydrocarbon solvents, mixtures of such solvents or other volatile organic solvent materials. The use of such solvents is disadvantageous in view of the toxicity and environmental problems inherent in such materials and, accordingly, the use of such solvents is in the process of being banned by international treaty or has been subjected to close scrutiny by various government agencies.
To replace these organic solvent materials for use in cleaning rosin flux residues from circuit assemblies, the present assignee has developed the ARMAKLEEN.RTM. cleaner which is an aqueous-based cleaner comprising alkali metal salts such as alkali metal carbonate and/or alkali metal bicarbonate salts, alkali metal silicates for use as a corrosion inhibitor as well as organic adjuvants such as surfactants to improve the efficacy of cleaning, anionic polymers to stabilize the silicate in solution as well as hydrotropes to maintain the surfactants in aqueous solution. ARMAKLEEN.RTM. cleaners are described, for example, in U.S. Pat. Nos. 5,234,505 and 5,234,506, which are hereby incorporated by reference herein. These aqueous-based cleaners have been found to be very effective for removing rosin flux residues and are substantially safer to use relative to the operator and the environment than the previous organic solvent-based cleaners.
Although the above-described aqueous-based cleaners are very effective in removing soldering rosin flux residues from the substrate of an electronic circuit assembly, it is continually desirable to provide improved compositions and methods for removing rosin flux residues from electronic circuit assemblies. In particular, the high SMT component densities make it even more difficult to ensure complete removal of flux residues.
The ARMAKLEEN.RTM. cleaners disclosed for example in U.S. Pat. No. 5,431,847 (which is hereby incorporated by reference herein) and in copending, commonly assigned U.S. application Ser. No. 08/417,883, filed Apr. 6, 1995 include nonionic alkoxylate and N-alkylpyrrolidone surfactants either alone, or preferably, in mixtures to enhance cleaning efficacy. While the ARMAKLEEN.RTM. aqueous-based cleaners have been shown to be very effective rosin flux removers, it is still desirable to provide such aqueous cleaners with a surfactant formulation which can reduce the surface tension thereof and enhance the flux-removing ability of the cleaner especially in view of the ever increasing component density on the boards.
During the manufacture of electronic circuit assemblies, the boards are plated, etched, handled by operators in assembly, coated with corrosive or potentially corrosive fluxes, and soldered. Thus, contamination other than from fluxes are present. The cleanliness of the electronic circuit assemblies, such as printed circuit boards or printed wiring boards, is generally regarded as being critical to their functional reliability. Accordingly, all ionic and nonionic contamination on a circuit assembly can lead to premature failure of the assembly by allowing short circuits to develop therein.
Fluoro-containing surfactants are known. However, the use of same in purely aqueous cleaners (no organic solvents) and the use of such cleaners to remove rosin flux is not believed to be known. A no-clean soldering flux containing a halide-free, water-soluble activator and a fluorinated surfactant in water is disclosed, e.g., in U.S. Pat. No. 5,297,721 (Schneider et al.). U.S. Pat. Nos. 5,128,057 and 5,308,402 (both to Bixenman et al.) disclose aqueous flux-removing cleaning agents composed of tetrahydrofurfuryl alcohol (THFA) and an amine, ester or ether activator. The patents to Bixenman et al. further disclose the addition of nonionic surfactants to the cleaner including fluorinated surfactants. The Bixenman et al cleaners are an attempt to replace the previously used chlorofluorocarbon solvents (CFCs) to clear circuit boards. While the cleaners of Bixenman are less volatile than CFCs, the presence of THFA in addition to relatively high levels of organic activators would lead to substantial increases in the biological oxygen demand (BOD) and chemical oxygen demand (COD) in waste streams. None of the above patents are directed to aqueous alkaline salt-based cleaners wherein the organic content is limited.
Accordingly, a primary object of this invention is to provide an improved aqueous alkaline salt-based cleaning composition for removing rosin flux residues from a substrate, preferably a substrate of an electronic circuit assembly, wherein the cleaning composition has a reduced surface tension and, thus, improved flux-removing capacity.
A further object of the present invention is to provide a method of removing rosin flux residues from a substrate, preferably a substrate of an electronic circuit assembly, by means of a cleaning composition which has a reduced surface tension and, thus, improved flux-removing capacity.
These and other objects which are achieved according to the present invention can be readily discerned from the following description.